1. Field of the Invention
The invention described is a design improvement of the cylindrical canister or respirator filter that is used in conjunction with a gas mask for individual protection against respiratory hazards. The canisters are used in military applications to protect soldiers against chemical warfare agents.
In civilian applications, respirators are used to protect workers in environments contaminated with toxic and/or noxious gases or vapors. Presently, military canisters and civilian respirators use a cylindrical geometry for the carbonaceous adsorbent bed.
This invention improves the problem of sacrificing protection time, against chemical and biological warfare agents, for pressure drop, in canister design. Pressure drop, or breathing resistance, is a measure of the difficulty one experiences in breathing through a gas mask canister. The deeper the carbon bed in the canister, the greater the protection against toxic agents. However, as bed depth increases, so too does the pressure drop. In the past, the depth of the carbon bed was shortened, at the expense of protection time, to lower the pressure drop of the canister thus making it easier for the user of the canister to breathe. The problem of maximizing protection, and minimizing pressure drop, has existed since canisters were first designed in World War I to protect soldiers against poison gas attacks.
2. Description of the Prior Art
The old ways of lowering pressure drop used the three techniques listed below.
1. Decrease the depth of the carbon bed.
2. Increase the particle size of the carbon in the bed.
3. Increase the diameter of the carbon bed.
The old ways of increasing protection time used the three techniques listed below.
4. Increasing the depth of the carbon bed.
5. Decrease the particle size of the carbon in the bed.
6. Impregnating the carbon in the canister or respirator bed with reactive chemicals.
The opposing guidance that 1, 2 give in relation to 4, 5 point out the give and take nature of the old ways of solving the conflicting problems of pressure drop and protection. The old ways of solving the problems of pressure drop and protection are unsatisfactory because each has a significant drawback.
Decreasing the bed depth will lower the pressure drop but it will also decrease the protection time against toxic agents for user of the canister or respirator. Increasing the bed depth increases the pressure drop by introducing more resistance for the air stream passing through the carbon bed off the canister. The more carbon particles the air stream must bypass, the greater the resistance to flow and hence, the greater the pressure drop and breathing resistance for the user.
Increasing the diameter of the bed will lower pressure drop. However, greatly increasing the size of the canister may restrict the movements or vision of the user. The velocity in a packed bed is equal to the total flow divided by the cross sectional area throughout which the flows passes. EQU Velocity=total flow/cross sectional area
Increasing the diameter of the canister lowers pressure drop because it lowers the velocity of the air stream passing through the carbon bed. An air stream with a lower velocity encounters less resistance than an air stream with a higher velocity.
Increasing the particle size of the carbon in the bed will lower the pressure drop, but it will also decrease the internal mass transfer rate. The internal mass transfer rate is a measure of the movement of the adsorbate (that which is to be absorbed i.e. toxic compounds) into the adsorbent (the medium into which the adsorbate adsorbs i.e. carbon). It is measured as a mass per unit time. For the purposes of protection against toxic agents or chemicals, the greater the internal mass transfer rate, the better. The internal mass transfer rate of the carbon particle is, among other things, dependent on the distance between the external surface of the particle and the internal adsorption sites in the pores of the carbon particle. The bigger the particle, the greater the distance between the external surface of the particle and the adsorption sites in the pore. The greater the distance, the longer it takes for any adsorbate, such as a toxic agent to move from the exterior of the particle to the adsorption site inside the particle. Hence, a bigger particle size results in a lower internal mass transfer rate, and, in many cases, a shorter protection time for the user of the canister.